Recently, there has been a growing interest on cars, such as a hybrid car and an electric car, which utilize electric energy with depletion of fossil fuel and deterioration of global environmental issues as the background thereof, and such cars have been practically used. Such cars using the electric energy are often provided with a power converter that steps down a voltage from a high voltage battery, configured to supply power to a motor configured to drive a wheel, and supplies the required power to low-voltage electric equipment. In general, a switching power supply device is used in the power converter that supplies power to the electric equipment such as an air conditioner, an audio, and a controller of the car.
Here, losses such as a copper loss and a fixed loss are generated in the power converter at the time of converting the power. The copper loss generated in the switching power supply device is proportional to the square of an output current. There is a case where two switching power supply devices are provided in parallel in order to improve conversion efficiency of a power converter. When the power converter is configured to include the switching power supply devices connected in parallel, since it is possible to halve each output current of the switching power supply devices, the copper loss is reduced, which enables significant improvement of conversion efficiency of the power converter.
In addition, the losses generated in the switching power supply device generally include the fixed loss (for example, an iron loss generated as a voltage is applied to a transformer) that does not depend on a magnitude of a load current in addition to the copper loss that is proportional to the square of the output current. Since the fixed loss is generated without depending on an output current value, a proportion occupied by the fixed loss in the entire loss becomes relatively great in a state where the output current value is small. In particular, when the electric equipment, such as the air conditioner, with a large load is used in the car, a load current thereof approaches a maximum output current value of the power converter thereof. On the contrary, when the above-described electric equipment with the large load is not used, the output current value becomes extremely small. That is, the power converter is driven in any state between the state with the large load and the state with the small load in many cases. Therefore, the conversion efficiency of the power converter is low when the output current value is small, and thus, consumption of a high voltage battery increases.
In this regard, there is a technique of stopping one of a plurality of switching power supply devices connected in parallel when an output current value is small. When the output current value becomes an arbitrary value or smaller, control is performed such that one of the switching power supply devices connected in parallel is stopped. Through this control method, an operation of one of the switching power supply devices is stopped when a load is small. Thus, the fixed loss of the switching power supply device decreases, and accordingly, the conversion efficiency of the power converter in the state with the small load is improved. For example, a power converter disclosed JP 2002-291247 A (PTL 1) is known as the power converter in which the above-described control is performed.